Synchronization in Wireless Sensors Networks Using balanced clusters Salim EL KHEDIRI1,3, Nejah Nasri1 1 Laboratory LETI, University of Sfax, National School of Engineers of Sfax,Route Soukra Km 3.5 B.P W 1173, TUNISIA
[email protected],
[email protected]
Abdennaceur Kachouri2 University of Gabes, ISSIG Higher Institute of industrial Systems Gabes CP 6011, TUNISIA
[email protected] 2
Abstract— The advent of smart devices and continuous expansions of smart environments make Wireless sensor networks (WSNs) an important part of our daily lives. The usage of a myriad of devices in self-organizing networks in a various fields such as home monitoring, medical and military etc. requires an efficient delivery of sensed information. For this necessity, a local clock of sensors nodes needs to be synchronized and up-keep timely synchronization between sensors, to ensue seamlessly communication with each other via radio links aimed at sharing and treatment of reliable information. In this paper, we present balanced Timing-sync protocol for sensor networks that aims at providing network-wide time synchronization in a sensor network. Our schemes work in two steps. In the first step, a hierarchical structure is established in the edges of this structure to establish a global time scale through the network. Ultimately all node in the network synchronize their clock to a reference node. We implement our algorithm on NS2. We argue that our algorithm roughly gives better performance as compared to the work in the same line of research like TPSN. Keywords—Sensors networks, time synchronization, clock drift.
I. INTRODUCTION In wireless sensor networks, the aim operation is data fusion, whereby data from each sensor is agglomerated to form a single meaningful result [1]. In general, the proposed technique for synchronization in sensor networks requires that all sensor nodes have a common time scale so that the central unit can coordinate and collaborate sensors to accomplish their tasks. However, it is difficult to maintain a common time scale for all sensors, so the IEEE 802.15.4 standard has not defined clearly the synchronization mechanisms. Wireless sensor networks (WSNs) can be applied to a wide range of applications in domains as diverse as medical, industrial, military, environmental, scientific, and home networks [2], [4]. Since the sensors in a WSN operate independently, their local clocks may not be synchronized with one another. This can cause difficulties when trying to integrate and interpret information sensed at different nodes. For instance, if a moving car is detected at two different times along a road, before we can even tell in what direction the car is going, the detection times have to be compared meaningfully. In addition, we must be able to transform the two time readings into a common frame of reference before
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Anne Wei3 Center for studies in Computer sciences and communication, CNAM,
[email protected]
estimating the speed of the vehicle. Estimating time differences across nodes accurately is also important in node localization. For example, many localization algorithms use ranging technologies to estimate inter-nodes distances; in these technologies, synchronization is needed for time-offlight measurements that are then transformed into distances by multiplying with the medium propagation speed for the type of signal used such as radio frequency or ultrasonic. There are additional examples where cooperative sensing requires the nodes involved to agree on a common time frame such as configuring a beam-forming array and setting a TDMA (Time Division Multiple Access) radio schedule [6]. These situations mandate the necessity of one common notion of time in WSNs. Therefore, currently there is a huge research interest towards developing efficient clock synchronization protocols to provide a common notion of time. The remainder of this paper is structured as follows: the previous work related to the subject of clock synchronization is described in section II, we describe the proposed synchronization algorithm followed by simulation results are presented to verify the superiority of our scheme in section III. The conclusion of our paper is given in section VI. II.
RELATED WORK
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Figure 1. Classification of time synchronization scheme As illustrated in figure 1 time synchronization algorithm can be roughly classified into three types: (a) simple unidirectional
broadcast, (b) receiver-receiver synchronization and (c) senderreceiver synchronization. First in the unidirectional reference broadcast method, with a simple method a reference node simply broadcasts a reference clock signal to other nodes. Which correct their time with the reference clock. This method is the oldest and simplest method for synchronization the network time. The flooding time synchronization protocol (FTSP) [2], [3] is the most known approach that is to achieve a local synchronization with local participating nodes. Assuming that each node has local clock synchronization errors, and can communicate despite the lack of reliability, the errors must be corrected with message exchange mechanism. FTSP synchronize time from a sender to multiple receivers which may be using a single radio message. This mechanism could ensure high accuracy between two sensors and keep synchronized communication. Typically, WSN (Wireless Sensor Network) operates in larger areas than the radius of a node. Therefore, the FTSP synchronizes multihop nodes. The root node is the only selected and dynamic node that maintains the overall time for all other nodes to synchronize their clocks. The nodes form a Ad-hoc structure to transfer the total time from the root to all nodes that keep (save) the initial phase of the tree that is more robust against failures of links between nodes and the dynamic topology change. Second, receiver-receiver synchronization, Elson and al [5] have proposed the RBS approach which uses the concept of receiver-receiver synchronization. The later has been, viewed as reference to several works in the same line of research (Synchronization Solution). The RBS synchronization mechanism is based on the exploitation of the nature of diffusion of wireless medium. With this property, the nodes in the transmission range of the same location in the intersection of two neighborhoods would be synchronized. Despite the advantages of elimination of major sources of indeterminism, transmitter receive the same message with a very low offset. Considering only the time for receipt of different receptors, the RBS protocol immediately eliminates two major sources of indeterminism involved in the transmission of messages, errors and the precision of synchronization that follows. The mechanism RBS has certain limitations: it requires that the reference receivers of messages transmitted by the reference to know the real time and the advanced channel time. The only source of indeterminism that interfere in RBS synchronization are the propagation and receipt time which shall exchange times of receptions. Third, sender-receiver, uses the round trip time of the message to correct the offset and propagation delay. In figure 2 we detail an example of the basic operation, which includes three sequential phases. First, node A send it’s local time at t1, and node B receives the message at time t2 and records it’s clock, then, time t2 is calculated as t2=T1+d+!, where d the propagation delay between two nodes, and ! is a clock offset denote between them. Finally, node A can calculate the clock offset and propagation delay between two nodes as below:
D= (T2-T1)+(T4-T3)/2 ! = (T2-T1)-(T4-T3)/2 Figure 2. Offset and propagation delay Several studies using this mechanism, include for example: Timing-sync Protocol for Sensor Network (TPSN) [2], [4], Arnewel and al have proposed an alternative approach to synchronization with the type Transmitter-receiver, TPSN is a hierarchical algorithm which works on two different phases: The discovery and synchronization phase. In the first phase, we give a network node level. The node that initiates the synchronization is called the root node with the value of level zero neighbors with n hops (n=1, 2, 3, K) have the value of level n. This process continues until all neighbors attribute their levels. In second phase, a pair wise synchronization is performed along the edges of the hierarchical structure up to a total synchronization of the tree constructed with the message exchange mechanism. III. PROPOSED APPROACH A. PROPOSED APPROACH Our approach is based on the ideas mentioned in [3], [4], when number of levels increases, local clock offset difference also increase. For this reasons our scheme based on balanced clustering into network to minimize complexity, and As shown in figure 3, the network was comprised of a root node, cluster head nodes and many simple nodes (non-clusterhead members).
Figure 3. Tree divided in balanced clustering Some of the assumptions made in clustered time synchronization in wireless sensor network are as following: ! ! ! !
The network composed by N sensor nodes deployed in square field and has formed cluster hierarchical topology. The root node is located inside the sensing field and their level predetermined at level-0. Nodes are location-unaware. The cluster head nodes are aware of its members and can directly communicate with them.
! !
The cluster head are aware of their parent cluster heads Each node is synchronized with its CH.
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c. Define the depth for each cluster Synchronization phase a. b.
Figure 4 describe the algorithm of the proposed synchronization approach:
Synchronization inter cluster Global synchronization
B. SIMULATION AND ANALYSES $.%
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